This invention relates to a method of determining voltage stability margin at local bus level and to applying the method to enhance under-voltage load shedding protection scheme. We named this new protection scheme as “voltage instability load shedding”.
Under Voltage Load Shedding (UVLS) has been used as an economic means of avoiding voltage collapse. Since load shedding results in high costs to electricity suppliers and consumers, this option is only used when all other means of avoiding voltage collapse are exhausted. UVLS sheds load in pre-defined blocks that are triggered in stages when local voltage drops to the pre-defined levels.
In most UVLS schemes, voltage magnitude is the only triggering criterion. However, past research has demonstrated that voltage magnitude alone is not a satisfactory indicator of the proximity to voltage instability under all circumstances. In fact, voltage stability is determined by the ability of the power system to supply and deliver reactive power. In actual systems, the computation of actual system PV curves may be very complicated due to the large number of generators, widespread applications of capacitor banks, uncertainty about the dynamic characteristics of system loads, and the variability of power flow pattern. In addition, operation of under load tap changers, the actual dynamic reactive capability of generators and accurate reactive reserve all affect the ability of the system to supply and deliver the reactive power. Therefore, determination of proper settings for UVLS schemes becomes a challenging task for system planners.
Moreover, modeling uncertainties post more challenges for system planners to determine the proper settings for UVLS schemes. Current settings of UVLS are determined by system planning engineers through extensive network analyses using computer simulation packages. However, simulated system behaviors do not usually coincide with actual measured system responses due to data and modeling issues. Inappropriate settings can result in unnecessary shedding or failure to detect the need for load shedding.